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2 Answers
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EEG and MEG measure two different but related signals. While EEG records the electric activity from the scalp, MEG records changes in the magnetic field induced by electric activity in the brain. Because of this, MEG is largely insensitive to sources oriented radially, but can pick up sources oriented tangentially to the cortex. Moreover, EEG is sensitive to volume currents (i.e., the spread of currents across the brain), while MEG less so [1].

Because of this distinction, while two components can have the same latency — let's say the face-specific N170 and the M170 — the sources originating them might be different. Naming the components differently stresses this important distinction.

Apart from making the distinction between recording techniques (talking about N170 and you refer to ERP), here is an explanation:

Naming convention refers more to neurophysiology than waveform (component) directions.

The standard tool modeling the EEG/MEG activity is a dipole. A dipole consists of a pair of a negative and positive charges and it generates both an electric and a magnetic field (see figure). You can see in the electric field panel that the positive activity is ahead of the positive charge, here the head of the arrow (on the contrary, the magnetic field is oriented on the side of the dipole). Therefore, a positivity in an ERP signal (P300 for example) corresponds to an increase of the dipole activity measured ahead of the positive charge (resp. negative for N400).

On the contrary, the magnetic main activity does not follow the main direction of the dipole but its perpendicular one. Therefore, we can't relate an increase in a magnetic field to the positive/negative direction of the dipole.

Here, a magnetic N170 would not be grounded on neurophysiological basis, but just in the fact that N only refers to a negative on-going measure.